The effect of gillnet capture on the metabolic rate of two shark species with contrasting lifestyles

Abstract

In this study we investigated the metabolic consequences of simulated gillnet capture in species with contrasting life-history characteristics, the gummy shark, Mustelus antarcticus and the Port Jackson shark, Heterdontus portusjacksoni. Pre-stress standard and routine metabolic rates (MR) of individuals of these species were measured and compared to the MR obtained after simulated gillnet capture. Standard MRs of M. antarcticus and H. portusjacksoni were 168.5 ± 61.9 mgO2 kg-0.67 h−1 and 144.1 ± 20.4 mgO2 kg-0.67 h−1, respectively. Routine MRs of M. antarcticus and H. portusjacksoni were 202.6 ± 63.5 mgO2 kg-0.67 h−1 and 166.4 ± 11.2 mgO2 kg-0.67 h−1, respectively. The simulated gillnet-capture treatment decreased the MR of M. antarcticus by 59.3% and increased that of H. portusjacksoni by 21.6%. Our results confirm that M. antarcticus is highly sensitive to gillnet capture, exhibiting significant variability in MR among individuals and also a high delayed mortality rate. We interpret the decrease in MR as a compensatory mechanism for reducing oxygen consumption to cope with the stress imposed by capture. Mortality in this species was associated with higher pre-stress MR, and lower post-stress MR. In contrast, H. portusjacksoni showed mild increases in MR. Their resilience to capture stress most likely relies on effective buccal pumping and lower basal MR. A mixed-effects model approach permitted us to identify the main sources of variation in the MR measured, which were the individual differences, treatment effects, species variations and delayed mortality. The comparative approach employed in this work allowed us to understand and provide reliable estimates of the effects of fishing on the MRs of these two species.